Gas separation is important in many industries and can be accomplished by conducting a mixture of gases over an adsorbent material that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. One of the more important types of gas separation technology is swing adsorption, such as pressure swing adsorption (PSA). PSA processes rely on the fact that under pressure gases tend to be adsorbed within the pore structure of a microporous adsorbent material or within the free volume of a polymeric material. The higher the pressure, the greater the amount of target gas component that is adsorbed. When the pressure is reduced, the adsorbed target component is released, or desorbed. PSA processes can be used to separate gases within a gas mixture because different gases tend to fill the micropore or free volume of the adsorbent to different extents. If a gas mixture, such as natural gas, is passed under pressure through a vessel containing a polymeric or microporous adsorbent that is more selective towards carbon dioxide, for example, than it is for methane, at least a fraction of the carbon dioxide is selectively adsorbed by the adsorbent, and the gas exiting the vessel is enriched in methane. When the bed reaches the end of its capacity to adsorb carbon dioxide, it is regenerated by reducing the pressure, thereby releasing the adsorbed carbon dioxide. It is typically then purged and repressurized and ready for another adsorption cycle.
While there are various teachings in the art with respect to new adsorbent materials, new and improved parallel channel contactors, and improved rapid cycle PSA (RC-PSA) equipment, none of these to date present a viable solution to the problem of producing good recovery of methane when the feed gas is at high pressure. This is a critical issue because natural gas is often produced at high pressures (30-700 bar) and it is preferred to operate the separation system at high pressure to avoid additional compression before transportation to the market. One problem in extending PSA processes to high pressures, especially with those streams containing large amounts of CO2, is that at the end of the adsorption step there can be significant amounts of product gas in the flow channels and void spaces. This can lead to poor recovery of the desired product and also to low purity product streams.
Achieving high recovery and high purity in separation processes at high pressures is especially beneficial in natural gas processing operations. Many natural gas fields contain significant levels of CO2, as well as other contaminants, such as H2S, N2, H2O mercaptans and/or heavy hydrocarbons that have to be removed to various degrees before the gas can be transported to market. It is preferred that as much of the acid gas (e.g., H2S and CO2) be removed from natural gas as possible, and some applications require high purity product gas with parts per million levels of contaminants to meet safety or operational specifications. In all natural gas separations, methane is the valuable component and acts as a light component in swing adsorption processes. Small increases in recovery of this light component can result in significant improvements in process economics and also serve to prevent unwanted resource loss.
Conventional commercial practices for removal of acid gases from natural gas are limited in reaching high recovery and high purity, especially when acid gas concentrations are greater than 30%, because these processes involve considerable energy input in the form of refrigeration, and they often require sizable equipment. For example, the conventional methods for removing up to 20 mole percent (mol %) to 30 mol % acid gases from natural gas streams include physical and chemical solvents. These processes require handling and inventory storage for solvent as well as significant energy consumption for recovering the solvent. For higher acid gas concentrations, some applications use bulk fractionation combined with technology like a Selexol physical solvent system which requires refrigeration and can result in extensive loss of heavy hydrocarbons to the acid gas stream.
Generally, simple PSA cycles can not take advantage of the kinetics of adsorption because the cycle times are long, and conventional PSA systems typically result in significant loss of methane with the acid gas stream. The relatively low product recovery along with the large size and cost of conventional PSA systems typically prohibits their use in large-scale natural gas processing applications. While various concepts have been proposed to enhance the performance of PSA systems, none have enabled separations at high pressure that provide the product purity and recovery required for natural gas processing. Therefore, a need exists in the art for improved processes to remove contaminants from feed streams, such as natural gas streams, at high pressure with high product purity and product recovery.